Banking giant UBS is coughing up $1.5 billion in fines — and two of its fat-cat traders face prison — after a US and UK investigation of the bank found “routine and widespread” manipulation of a benchmark interest rate.

The scheme, the largest interest-rate rigging scandal ever, caused lopsided outcomes in more than $450 trillion in annual loans made around the world for everything from credit cards and mortgages to college loans and business deals, federal prosecutors said yesterday.

The criminal complaint against the Swiss bank and the arrest of two former UBS traders show authorities have amped up their financial-sector probes.

The crackdown on rampant corruption in banking’s back offices came in a landmark joint probe by authorities in at least five nations, which charge UBS is a major culprit in notorious manipulations of the key Libor rate.

Banks set the rate each day amongst themselves, using a panel of their own representatives and assumptions of short-term lending demand.

The surprising scope of the alleged schemes opens the floodgates for worldwide litigation from financial institutions — which unknowingly lost millions on the wrong side of rigged lending.

Prosecutors said that more than 45 traders and brokers at the bank, along with scores of outside currency middlemen, conspired almost daily, with bribes of typically $20,000 trading hands to influence Libor rates.

Each illicit pact set up among the traders and brokers was aimed at profiting on their positions from tiny moves in the rate.

Even a brief 1/100th of a percent move in the Libor translated to profits of about $459,000 for one of the UBS traders accused in the crackdown.

That cheated counter-parties, the complaint said.

“UBS manipulated one of the cornerstone interest rates in our global financial system,” said Assistant Attorney General Lanny A. Breuer.

“The scheme alleged is epic in scale, involving people who have walked the halls of some of the most powerful banks in the world,” he said.

Yesterday’s actions included the first criminal charges made in the probes, leveled against Tom Alexander Hayes, 33, and Roger Darin, 41.

London police picked up Hayes; Darin is still at large.

The US Justice Department filed to extradite the pair to face trial here.

Numerous outside brokers were also involved, said enforcement chief David Meister of the Commodity Futures Trading Commission.

“These brokers are supposed to be honest middlemen,” he said. “The brokers here were anything but honest.”

The government complaint quoted one broker telling a UBS trader regarding an illicit pact’s profits: “‘Mate, you’re getting bloody good at this Libor game. Think of me when yur on yur yacht in monaco wont yu.’ ”

Prosecutors said more people were to be named in the ongoing probe.

As many as 40 employees of UBS have left the bank over the case, prosecutors said.

In exchange for a deferred prosecution of the company, UBS agreed to plead guilty to civil violations at the unit in Japan, leaving the parent to pick up the hefty tabs and suffer the black eye.